This invention relates to piston rings for internal combustion engines, and more particularly to a compression ring for a reciprocating piston internal combustion engine. The compression rings of a reciprocating piston engine provide a sliding seal between the piston and cylinder wall to prevent combustion gases from leaking past the piston, ie, gas blow-by. More particularly, the present invention provides a compression ring which has a circumferential relief of L-shaped cross section provided at its upper-outer (diameter) portion to reduce the bearing contact area of the ring with the cylinder wall, and a corresponding circumferential relief at its upper-inner (diameter) portion to help balance the ring against twist and help to properly seat it in its seating groove.
It is known to the prior art to provide compression rings whose outer face is beveled or chamfered to provide an upwardly and inwardly inclined outer peripheral surface whereby the bearing contact area of the ring with the cylinder wall within which it is disposed is reduced. Such reduction helps to diminish bearing friction of the ring against the cylinder wall. It is further known to provide a bevel or chamfer on the upper inside peripheral surface of such rings. U.S. Pat. No. 3,337,938 of H. F. Prasse et al, assigned to the assignee of this application, shows such an arrangement, in the context of a torsion ring having a twisted configuration as shown in FIG. 9 of Prasse et al. The provision of symmetrical cut-outs to avoid unintentional torsion twisting of rings is shown in U.S. Pat. No. 2,423,017.
One difficulty with sloping contact faces of the compression ring is that wear of the bearing face reduces the effective back-pressure surface against which combustion gas pressure may act during the compression and power strokes to help balance the forces tending to thrust the ring against the cylinder wall.
Attempts to reduce the bearing surface contact of the ring against the cylinder wall simply by reducing the axial width of the ring and employing the full outer face ring surface in bearing contact are handicapped because such structure eliminates the back-pressure surface and because there is a limit to how thin the ring can be made. That is, sound ring castings cannot be reliably attained for rings of less than about 0.060 inch of axial width.
Another difficulty of prior art rings is that unintended and disadvantageous torsional twisting of the ring is caused by unbalanced ring cross section profiles, so that both the effect of gas pressure acting on the ring and internal stresses set up in the ring by the manufacturing process tend to twist the ring out of a desired flat seating engagement with its associated groove, resulting in poor sealing and consequent gas blow-by.